In communication and control systems, delay lines are used to store a signal for a discrete period, and to supply that signal at an output point at the end of the period. This period between the time the signal is input and the time the signal is output is called the delay time. A tapped delay line is a variation in which the signal is supplied at several different output points, the distance between successive output points being that distance which will provide a delay time equal to the period of the fundamental frequency of the device. Tapped delay line applications include use in correlating and convoluting devices.
If an input signal which varies as some function of time is supplied to the input of the tapped delay line, the tapped delay line may be used to filter selected frequencies from the input signal. By summing the outputs of the tapped delay line, the device will attenuate any input signal content other than of the fundamental frequency, or that fundamental frequency's harmonics.
For example, by making the period between the several output points five microseconds, a fundamental frequency having a five microsecond period will be provided at the summed output, along with the various harmonics of that fundamental frequency. For the five microsecond period, the fundamental frequency would be 200 KHz. When a tapped delay line is used in this way to pass some frequencies and reject others, it is acting as a transversal filter.
There are three basic types of delay lines which may act as transversal filters. The first type comprises a group of devices utilizing wave phenomena in which waves may reinforce to cause the device to act as a transversal filter. The second type utilizes a considerable length of transmission medium with the signal being removed at taps which are separated by equal lengths of the transmission media, these signals then being summed to provide the desired system output. The third type of system recirculates the signal to allow the desired fundamental and harmonic frequencies to reinforce themselves, with the resulting output being in effect already summed by the recirculating action.
The first group are those devices utilizing wave phenomena to obtain a similar output. Many of these devices use coaxial cables or acoustic wave guides for the transmission and summation of signals. The time delay in these devices is produced because of the time a signal takes to travel through the delay line from the input end to the output end. Portions of the signal will be reflected and will propagate from the output end back to the input end, where they are reflected to the output end again. Where an input function is continuously supplied, these devices will reinforce the signal at some fundamental frequency and that frequency's harmonics, while attenuating all other frequencies, and will provide at the output end a signal comprising the fundamental and harmonic frequency content of the input signal.
The coaxial cable delay line is the most common of these devices, and microwave signals may be stored in coaxial cables for some period of time. The main disadvantage of coaxial cables is that they have a fairly limited bandwidth, making coaxial cable useless at high frequencies and with short pulses.
At frequencies above 100 MHz, coaxial cable is subject to severe loses, and high frequencies will thus not be transmitted accurately. In addition, if the pulse being transmitted is of extremely short duration, e.g., one nanosecond, it will be degraded and spread out rather than remaining sharp.
Coaxial cable is also susceptible to electromagnetic interference, particularly when the frequencies being transmitted are relatively high. Finally, fairly long lengths of coaxial cable may be necessary to allow the device to function as a transversal filter at lower frequencies, and such devices are quite bulky and also fairly expensive.
Another type of device utilizing wave phenomena is the acoustic delay line device. There are two types of acoustic delay lines: bulk-wave devices, and surface-wave devices. Bulk-wave devices use the principle of compression and rarefaction of the bulk material, and have input and output transducers at the ends of the bulk material. Bulk-wave devices, unfortunately, require large bias voltages and thus present a heat dissipation problem, so that only pulsed operation of bulk-wave devices is feasible.
Surface-wave devices operate with acoustic surface waves, and utilize charge carriers in a thin film of silicon placed adjacent to an insulating piezoelectric crystal. Surface acoustic wave devices operating at UHF frequencies have been developed and operate with multiple taps installed in the transmission medium. The main disadvantage of such devices is that their upper operational frequency limit is approximately one GHz, and it is desirable to have a transversal filter which is operable at higher frequencies. Therefore, it can be seen that devices utilizing wave phenomena are not very satisfactory when used as transversal filters at high frequencies.
Tapped delay lines having a number of taps at different lengths of the transmission medium are generally of two types: electrical, and optical fiber. The electric tapped delay line is simply a long segment of wire with outputs at multiple points along this wire. The fundamental frequency of such a tapped delay line is selected by a uniform length of wire between outputs, the time an electrical impulse takes to travel from one output to the next such output being the period of the fundamental frequency. Such devices are fairly bulky and expensive, since the requirement of having hundreds or possibly even thousands of outputs means that fairly large lengths of wire will be needed. Such devices also have a severe limitation in their operational bandwidth, and are generally not operable at high frequencies or in an environment having a not insubstantial amount of electromagnetic interference.
The optical fiber type of tapped delay line has significant advantages in that it is not susceptible to electromagnetic interference, is operable at relatively high frequencies, and optical fiber is substantially less bulky than wire. However, in order to obtain performance over a wide range of frequencies from existing optical fiber devices, hundreds or even thousands of optical taps must be utilized. This can be done with current technology by fabricating discrete couplers at each tapping point. Such a system is not really feasible in that it is extremely difficult to construct, quite expensive, and would be difficult to accomplish without lowering the signal level substantially. However, the concept of sampling the signals in an optical fiber at discrete intervals is an important one, and will be utilized by the present invention.
Another type of optical fiber tapped delay line is one which uses multiple segments of optical fiber, each segment being a standard length longer than the preceding segment, the standard length being the length through which light travels in one period of the fundamental frequency. The signal being analyzed is introduced into these segments simultaneously, and the outputs of each of these segments is summed to produce an output signal comprising the fundamental and harmonic frequency content of the input signal.
While this device accomplishes the desired result, it presents the substantial problem of necessitating an input signal to be simultaneously supplied to hundreds or even thousands of optical fiber segments. Such a device would be difficult to construct, and would also be somewhat bulky.
Each of the above optical fiber devices also presents the disadvantage of not being able to change the tap weighting dynamically without extensive modifications to the device. In other words, once such a device is constructed, the relative weighting of various outputs which are to be summed may not be changed in order to tailor the output signal of the device.
The second type of tapped delay line is a recirculating memory type device, such as that described in U.S. Pat. No. 4,473,270, entitled "Splice Free Fiber Optic Recirculating Memory," and assigned to the assignee of the present invention. Such device operates in a way quite similar to the wave phenomena devices described above--a signal recirculates through a fiber optic loop in the recirculating memory devices, with the output of the device being a summed signal comprising the system-set fundamental and harmonic frequency content of the input signal, with all other frequencies being attenuated. The fundamental frequency has a period equal to the time taken for a signal to make one circulation through the loop.
Such devices have the advantages of being operable at high frequencies, being unaffected by electromagnetic interference, and being fairly compact. However, when used as a transversal filter, such devices have several disadvantages. First, in order to obtain an output signal of a usable level, recirculating memory devices can only provide a fairly limited number of circulations before the signal level drops below the usable level. This is a particular problem since it is desirable to have hundreds or even thousands of points at which the signal is taken and summed in order to obtain an accurate and sharply defined passband. A second substantial disadvantage of such devices is that there is no way to change the dynamic weighting of the output signals taken at various points before they are summed, since the summing is done within the device. Finally, since recirculating memory devices have a fixed loop length, there is a limitation on the length of signals input to such devices.
Therefore, there is a need for a device which has a large number of discrete taps, each tap being capable of removing the signal at some discrete point in the delay line. Each of the tapped outputs should be discrete, so that dynamic weighting of the outputs may be accomplished in order to tailor the resulting output of the system when the signals are summed. For example, by weighting the various output signals, a more nearly rectangular band for a transversal filter may then be obtained.